The definition of adult stem cell niches in the bone marrow (BM) and their regulation by extrinsic factors is a top research priority in tissue engineering and skeletal regenerative medicine, as well as a critical aspect of hematopoietic function and its reconstitution after marrow ablation therapy. Unlike the wealth of information regarding the cell types, cell-cell interactions and soluble signals that specify functionally discrete BM microenvironments, significantly less is known about the role of the extracellular matrix (ECM) in niche function to the detriment of developing more effective stem cell-based therapies. Our preliminary findings are the first to indicate involvement of fibrillin-1, a unique ECM protein that regulates the spatial organization and physical properties of tissues as well as the bioavailability of endogenous (local) TGF family signals. This discovery raises the exciting possibility that a structural component of the marrow matrix controls the production of bone, blood and immune cells by determining the physical microenvironment of resident stem cells and by coordinating regulatory signals within it. Fibrillin-1 is the mutated protein in Marfan syndrome (MFS), whose pleiotropic manifestations include progressive bone loss (osteopenia). We previously demonstrated that osteopenia in MFS mice reflects perturbed bone remodeling due to impaired calibration of local anabolic and catabolic signals. Ongoing investigations have implied that fibrillin-1 is an essential component of the BM microenvironment that specifies the performance of mesenchymal stem cells (MSC) and hematopoietic stem cells (HSC). The goal of this high risk/high reward R21 application is to answer the following two questions: (a) What are the phenotypic consequences of fibrillin-1 deficiency for BM-based osteogenesis and hematopoiesis? ; (b) Does fibrillin-1 coordinate MSC-supported HSC performance? Accordingly, we propose to: (Aim 1) Characterize impaired MSC activity and bone loss progression in mice lacking fibrillin-1 in the appendicular skeleton and to provide a full account of hematopoietic abnormalities in these mutant animals; and (Aim 2) Validate the role of fibrillin-1 in coordinating MSC-supported HSC differentiation using mice with conditional Fbn1 inactivation in a specific stromal cells population. By demonstrating that fibrillin-1 is an indispensable functional component of BM niches, the experiments will establish the basis for future interrogation of the mechanism mediating fibrillin-1 regulation of marrow niches; and by implicating fibrillin-1 in immune system function, they may also lead to a radically new understanding of the cellular events responsible for aortic disease progression in MFS with unanticipated opportunities for therapy. Overall, this highly innovative proposal is expected to yield novel insights into age-related bone loss, advance fundamental knowledge of stem cell biology, impact a variety of translational applications in regenerative medicine, and perhaps improve the clinical management of life- threatening aortic manifestations in MFS.